Real-time online determination of caustic in process scrubbers using near infrared spectroscopy and chemometrics

ABSTRACT

Disclosed herein are processes, apparatuses, and systems for monitoring and/or controlling caustic concentrations in caustic scrubbers. In various aspects, the processes, apparatuses, and systems comprise a real-time online method for measuring the concentration of caustic in process scrubbers wherein a probe is coupled to a spectrometer; collecting absorption data with wavelength range from about 1000 to about 2000 nm. In a further aspect, this technique tracks the use and recharge of caustic in process scrubbers. This abstract is intended as a scanning tool for purposes of searching in the particular art and is not intended to be limiting of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of U.S. Provisional Application No.61/691,214, filed on Aug. 20, 2012, which is incorporated herein byreference in its entirety.

FIELD OF INVENTION

The present invention relates to caustic scrubbers. More specifically,the invention relates to processes, apparatuses, and systems formonitoring and/or controlling caustic concentrations in causticscrubbers.

BACKGROUND

Process scrubbers are prevalent in chemical industry. In polycarbonate(PC) production, caustic scrubbers are frequently used to remove acidicgases by converting them to salt, water, and carbon dioxide. This istypically done by mixing the acidic effluents from the reactors withcaustic in a column via a counter flow system. As the acidic effluentsreact with the caustic, this operation depletes the caustic strength. Tomaintain adequate levels of caustic, the concentration of caustic isnormally monitored through a manual sampling, and the causticconcentration is determined with a manual acid-base titration. Suchcaustic strength analysis is typically done on a regular periodicitythroughout the day. Depending on the titration results, the causticconcentration can be adjusted by adding fresh caustic solution to thescrubber. A long time period between the manual caustic measurements canresult in inefficient operation of the scrubbers and possible processsafety issues. Thus, there is a need for a real-time online solution toensure safe operation and efficient use of caustic. This need and otherneeds are satisfied by the disclosed invention.

SUMMARY OF THE INVENTION

As described in more detail herein, the present invention providesprocesses, apparatuses, and systems for monitoring and/or controllingcaustic concentrations in caustic scrubbers.

In one aspect, the invention relates to a monitoring process comprisingthe steps of: providing a spectroscopy probe coupled to a causticscrubber; and measuring the near-infrared absorbance in the scrubber.

In a further aspect, the invention relates to a caustic scrubbercomprising a near-infrared spectroscopy probe coupled thereto.

In a further aspect, the invention relates to a polycarbonate productionsystem comprising the disclosed caustic scrubber and/or the disclosedmonitoring process.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate several aspects and together withthe description serve to explain the principles of the invention.

FIG. 1A is a schematic of a caustic scrubber.

FIG. 1B is a schematic of a caustic scrubber.

FIG. 2 shows representative data comparing real-time near infrared (NIR)and titration methods on caustic scrubber samples.

FIG. 3A shows representative data comparing online real-time nearinfrared (NIR) and titration caustic values.

FIG. 3B shows a representative dataset within the 14 day period showingthe frequency of sampling for the online real-time NIR method.

FIG. 4 shows representative data for real-time online near infrared(NIR) caustic monitoring.

FIG. 5 shows representative data for the perturbation of the real-timeonline near-infrared (NIR) caustic concentration within 24 hour period.

Additional advantages of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or can be learned by practice of the invention. Theadvantages of the invention will be realized and attained by means ofthe elements and combinations particularly pointed out in the appendedclaims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

DETAILED DESCRIPTION

The present invention can be understood more readily by reference to thefollowing detailed description of the invention and the Examplesincluded therein.

Before the present compounds, compositions, articles, systems, devices,and/or methods are disclosed and described, it is to be understood thatthey are not limited to specific synthetic methods unless otherwisespecified, or to particular reagents unless otherwise specified, as suchcan, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular aspects only andis not intended to be limiting. Although any methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, example methods andmaterials are now described.

Moreover, it is to be understood that unless otherwise expressly stated,it is in no way intended that any method set forth herein be construedas requiring that its steps be performed in a specific order.Accordingly, where a method claim does not actually recite an order tobe followed by its steps or it is not otherwise specifically stated inthe claims or descriptions that the steps are to be limited to aspecific order, it is no way intended that an order be inferred, in anyrespect. This holds for any possible non-express basis forinterpretation, including: matters of logic with respect to arrangementof steps or operational flow; plain meaning derived from grammaticalorganization or punctuation; and the number or type of embodimentsdescribed in the specification.

Throughout this application, various publications are referenced. Thedisclosures of these publications in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the state of the art to which this pertains. The referencesdisclosed are also individually and specifically incorporated byreference herein for the material contained in them that is discussed inthe sentence in which the reference is relied upon. Nothing herein is tobe construed as an admission that the present invention is not entitledto antedate such publication by virtue of prior invention. Further, thedates of publication provided herein may be different from the actualpublication dates, which can require independent confirmation.

Definitions

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present invention, example methods andmaterials are now described.

As used in the specification and the appended claims, the singular forms“a,” “an” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a ketone” includesmixtures of two or more ketones.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent ‘about,’ it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint. It is also understood that there are a number of valuesdisclosed herein, and that each value is also herein disclosed as“about” that particular value in addition to the value itself. Forexample, if the value “10” is disclosed, then “about 10” is alsodisclosed. It is also understood that each unit between two particularunits are also disclosed. For example, if 10 and 15 are disclosed, then11, 12, 13, and 14 are also disclosed.

As used herein, the terms “optional” or “optionally” means that thesubsequently described event or circumstance can or cannot occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not. For example, the phrase“optionally substituted alkyl” means that the alkyl group can or can notbe substituted and that the description includes both substituted andunsubstituted alkyl groups.

Disclosed are the components to be used to prepare the compositions ofthe invention as well as the compositions themselves to be used withinthe methods disclosed herein. These and other materials are disclosedherein, and it is understood that when combinations, subsets,interactions, groups, etc. of these materials are disclosed that whilespecific reference of each various individual and collectivecombinations and permutation of these compounds cannot be explicitlydisclosed, each is specifically contemplated and described herein. Forexample, if a particular compound is disclosed and discussed and anumber of modifications that can be made to a number of moleculesincluding the compounds are discussed, specifically contemplated is eachand every combination and permutation of the compound and themodifications that are possible unless specifically indicated to thecontrary. Thus, if a class of molecules A, B, and C are disclosed aswell as a class of molecules D, E, and F and an example of a combinationmolecule, A-D is disclosed, then even if each is not individuallyrecited each is individually and collectively contemplated meaningcombinations, A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F are considereddisclosed. Likewise, any subset or combination of these is alsodisclosed. Thus, for example, the sub-group of A-E, B-F, and C-E wouldbe considered disclosed. This concept applies to all aspects of thisapplication including, but not limited to, steps in methods of makingand using the compositions of the invention. Thus, if there are avariety of additional steps that can be performed it is understood thateach of these additional steps can be performed with any specific aspector combination of aspects of the methods of the invention.

References in the specification and concluding claims to parts byweight, of a particular element or component in a composition orarticle, denotes the weight relationship between the element orcomponent and any other elements or components in the composition orarticle for which a part by weight is expressed. Thus, in a compoundcontaining 2 parts by weight of component X and 5 parts by weightcomponent Y, X and Y are present at a weight ratio of 2:5, and arepresent in such ratio regardless of whether additional components arecontained in the compound.

A weight percent of a component, unless specifically stated to thecontrary, is based on the total weight of the formulation or compositionin which the component is included. For example if a particular elementor component in a composition or article is said to have 8% weight, itis understood that this percentage is relation to a total compositionalpercentage of 100%.

As used herein “means for determining caustic concentration” refers tothe use of a chemometric model to predict caustic strength of theanalyte stream from the NIR spectra data. Briefly, NIR spectra arecollected on solutions for which the caustic strength has beendetermined by an off-line titration method. This caustic strength datais paired up with its corresponding NIR spectrum and used together inconstructing a chemometric model. Numerous spectra with correspondingcaustic concentration data are used in constructing the model. Inpredicting the caustic strength, the sample NIR spectra are fed into themodel. The model then predicts the concentration of caustic based uponthat NIR spectrum.

As used herein “means to automatically introduce caustic into thescrubber to maintain a concentration range” refers to the interface ofthe means for determining caustic concentration with a plant controlsystem wherein when the caustic concentration in the analyte streamfalls below the lower control limit as determined using the means fordetermining caustic concentration, then a signal is conveyed to theplant control system to dispense into the scrubber stream fresh causticsolution. The amount of fresh caustic solution to dispense, as conveyedto the plant control system, is related to the difference between thecaustic strength as determined by the means for determining causticconcentration and the lower control limit.

Each of the materials disclosed herein are either commercially availableand/or the methods for the production thereof are known to those ofskill in the art.

It is understood that the compositions disclosed herein have certainfunctions. Disclosed herein are certain structural requirements forperforming the disclosed functions, and it is understood that there area variety of structures that can perform the same function that arerelated to the disclosed structures, and that these structures willtypically achieve the same result.

Near-Infrared (NIR) Spectroscopy

Near-infrared (NIR) is a widely used analytical measurement tool inchemical, pharmaceutical, petroleum, and agricultural industries. Thetechnique requires little or no sample preparation and isnondestructive, reagentless, simple, and fast. In addition, NIRspectroscopy exhibits the capability to extract quantitative informationof several species within a sample from a measured spectrum therebymaking this approach ideal for multicomponent determination of complexmatrixes.

In one aspect, the present invention provides a method to reduceanalysis cycle time, improve the safety of scrubber operations, andallows for the efficient use of cause via an online spectroscopictechnique as disclosed herein.

Near infrared (NIR) spectroscopy covering the 1000-2500 nm (10000-4000cm⁻¹) wavelength region is proposed for direct measurement of thecaustic scrubber stream. Briefly, a probe is inserted into the processstream, the probe is connected to a spectrometer via fiber optic cables,and NIR spectra acquired from the probe. Various probes suitable for usein the present invention are commercially available, including, forexample, the Guided Wave O-Ring Single Sided Transmission (O-SST) probe.Suitable NIR spectrophotometers for use in the methods of the presentinvention, including, but not limited to, Bruker Matrix-F FT-NIRspectrometer, Thermo Antaris MX FT-NIR spectrometer, ABB FTPA 2000FT-NIR spectrometer, and Foss NIR XDS Process Analyzer. The NIR methodsof the present invention is advantageous compared to prior art methodsbecause it is non-destructive, requires no sample collection orpreparation, and provides real-time measurements.

The present NIR spectroscopic method is calibrated using known standardcaustic concentrations. The known caustic concentrations andcorresponding NIR spectra are coupled and a model was developed thatrelates the spectra to the caustic concentration. The model is achemometric based method known as partial least squares (PLS) regressionand allows determination of caustic concentration in about 60 sec orless.

The chemometric based PLS calibration uses a range of wavelengths inconjunction with myriad spectral pretreatment algorithms to construct amore accurate and reproducible model. The use of wavelength range andspectral pretreatment minimizes the influence of other chemical speciespresent in matrix (such as carbonates, salts, and methlylene chloride)and process factors (temperature and flow rate).

Real-time online measurements of caustic in process scrubber streammakes the caustic analysis much safer compared to prior art describedearlier and enables significant economic benefit by efficientlyutilizing the caustic in the process scrubber. This is realizedespecially if the data is used immediately in a distributed controlsystem whereby the digital data from the NIR model is used to controlthe caustic level in the circulation tank. Monitoring processes

In one aspect, the invention relates to a monitoring process comprisingthe steps of: providing a spectroscopy probe coupled to a causticscrubber; and measuring the near-infrared absorbance in the scrubber. Ina further aspect, coupling is via direct contact. In a further aspect,coupling is via fiber optic.

In a further aspect, a NIR transmission probe is inserted in a side loopof the main scrubber column. In a still further aspect, online NIRspectra are acquired in real-time. In a yet further aspect, the causticscrubber is part of a polycarbonate production system. In an evenfurther aspect, the process further comprises the step of determiningcaustic concentration in the scrubber from the measured absorbance. In astill further aspect, the determining step is a chemometric-based methodusing partial least squares (PLS) regression which correlates the NIRspectra to reference titration values.

In a further aspect, the caustic scrubber is part of a polycarbonateproduction system. In a still further aspect, the process furthercomprises the step of determining caustic concentration in the scrubberfrom the measured absorbance. In a yet further aspect, the determiningstep is a chemometric-based method with spectral averaging over a timeperiod of from about 15 seconds to about 5 minutes.

In a further aspect, caustic is sodium hydroxide. Suitable basicmaterial includes alkali hydroxide, ammonia or ammonium hydroxide. In afurther aspect, both caustic and sodium carbonate are measured.

In a further aspect, the method further comprises the step ofintroducing caustic into the scrubber to maintain a concentration rangeof from about 2 wt % to about 25 wt % caustic. In a yet further aspect,the concentration range is from about 5% to about 15%. In a stillfurther aspect, the concentration range is from about 6% to about 8%. Ina further aspect, the caustic is introduced automatically in response tothe measured absorbance to maintain the concentration range.

In a further aspect, the absorbance is measured from about 1000 nm toabout 2000 nm. In a further aspect, the absorbance is measured fromabout 1700 nm to about 2300 nm.

In a further aspect, the Reynolds Number of the fluid flow in thescrubber is less than 1500. In a further aspect, the Reynolds Number ofthe fluid flow in the scrubber is greater than 4000.

In a further aspect, the invention relates to a process for monitoringand controlling caustic concentration in a caustic scrubber of apolycarbonate production system, the process comprising the steps of:coupling, via fiber optic, to a caustic scrubber; measuring thenear-infrared absorbance from about 1000 nm to about 2000 nm in thescrubber; determining caustic concentration in the scrubber from themeasured NIR spectra via a chemometric-based method using partial leastsquares (PLS) regression; and introducing, in response to the measuredcaustic content by NIR , caustic into the scrubber to maintain aconcentration range of from about 2 wt % to about 25 wt %.

Caustic Scrubbers

Waste streams produced in various chemical processes can contain acidiceffluent. It is desirable to lower the level of acidity before it isintroduced into the atmosphere in order to minimize the release ofpollutants to the atmosphere.

In various aspects, the caustic is introduced in response to themeasured absorbance to maintain the concentration range. In a furtheraspect, the caustic is introduced in response to the measured absorbancein accordance with a pre-determined set point value. In a still furtheraspect, the caustic is introduced automatically into the scrubber. In ayet further aspect, the automatic introduction of caustic is inaccordance with a pre-programmed algorithm.

In one aspect, the invention relates to a caustic scrubber comprising anear-infrared spectroscopy probe coupled thereto. In a further aspect,the probe is coupled to the scrubber via direct contact. In a furtheraspect, the probe is coupled to the scrubber via fiber optic. In a stillfurther aspect, the method further comprises means to automaticallyintroduce caustic into the scrubber. In a further aspect, the causticscrubber is part of a polycarbonate production system.

In a further aspect, the method further comprises the step ofintroducing caustic into the scrubber to maintain a concentration rangeof from about 2 wt % to about 25 wt % caustic. In a still furtheraspect, the concentration range is from about 2 wt % to about 20 wt %.In a yet further aspect, the concentration range is from about 2 wt % toabout 15 wt %. In an further aspect, the concentration range is fromabout 2 wt % to about 14 wt %. In a still further aspect, theconcentration range is from about 2 wt % to about 12 wt %. In a yetfurther aspect, the concentration range is from about 2 wt % to about 10wt %. In an even further aspect, the concentration range is from about 2wt % to about 8 wt %. In a still further aspect, the concentration rangeis from about 2 wt % to about 6 wt %.

In a further aspect, the method further comprises the step ofintroducing caustic into the scrubber to maintain a concentration rangeof from about 5 wt % to about 15 wt %. In a still further aspect, theconcentration range is from about 5 wt % to about 14 wt %. In a yetfurther aspect, the concentration range is from about 5 wt % to about 13wt %. In an even further aspect, the concentration range is from about 5wt % to about 12 wt %. In a still further aspect, the concentrationrange is from about 5 wt % to about 11 wt %. In a yet further aspect,the concentration range is from about 5 wt % to about 10 wt %. In aneven further aspect, the concentration range is from about 5 wt % toabout 9 wt %. In a still further aspect, the concentration range is fromabout 5 wt % to about 8 wt %.

In a further aspect, the method further comprises the step ofintroducing caustic into the scrubber to maintain a concentration rangeof from about 6 wt % to about 24 wt %. In a still further aspect, theconcentration range is from about 6 wt % to about 22 wt %. In a yetfurther aspect, the concentration range is from about 6 wt % to about 20wt %. In an even further aspect, the concentration range is from about 6wt % to about 18 wt %. In a still further aspect, the concentrationrange is from about 6 wt % to about 16 wt %. In a yet further aspect,the concentration range is from about 6 wt % to about 14 wt %. In aneven further aspect, the concentration range is from about 6 wt % toabout 12 wt %. In a still further aspect, the concentration range isfrom about 6 wt % to about 10 wt %. In a yet further aspect, theconcentration range is from about 6 wt % to about 8 wt %.

In a further aspect, the method further comprises the step ofintroducing caustic into the scrubber to maintain a concentration rangeof from about 50% to about 100% of the maximum upper level of caustic.In a still further aspect, the method further comprises the step ofintroducing caustic into the scrubber to maintain a concentration rangeof from about 60% to about 100% of the maximum upper level of caustic.In a yet further aspect, the method further comprises the step ofintroducing caustic into the scrubber to maintain a concentration rangeof from about 70% to about 100% of the maximum upper level of caustic.In an even further aspect, the method further comprises the step ofintroducing caustic into the scrubber to maintain a concentration rangeof from about 50% to about 75% of the maximum upper level of caustic.

In a further aspect, the caustic is introduced automatically in responseto the measured absorbance to maintain the concentration range.

In a further aspect, the invention relates to a caustic control systemin a polycarbonate production system, the scrubber comprising: a causticscrubber; a spectroscopy probe coupled to the scrubber via fiber optic,wherein the probe is capable of measuring the near-infrared absorbancein the scrubber; means for determining caustic concentration in thescrubber from the measured absorbance; and means to automaticallyintroduce caustic into the scrubber to maintain a concentration range offrom about 2% to about 25%. In a still further aspect, the concentrationrange is from about 5 wt % to about 15 wt %.

In a further aspect, the caustic control system further comprises asignal interface from means to automatically introduce caustic into thescrubber comprises to the means for determining caustic concentration inthe scrubber to the means to automatically introduce caustic into thescrubber. In a still further aspect, the means to automaticallyintroduce caustic into the scrubber comprises a programmed logiccontroller/distributed control system (“PLC/DCS”), wherein an outputsignal from the PLC/DCS interfaces with a means to introduce causticinto the scrubber, e.g. an electromechanical valve.

Alternatively, the addition to fresh caustic solution may be carried outentirely by manual operation. That is, based on information provided bythe means for determining caustic concentration, a human operator canadd fresh caustic solution to the scrubber by opening a valve thatallows fresh caustic to flow into the scrubber. The amount of freshcaustic solution added being determined by the difference between thecaustic concentration as determined by the means for determining causticconcentration and the lower control limit. Polycarbonate productionsystems

In one aspect, the invention relates to a polycarbonate productionsystem comprising the disclosed caustic scrubbers. In a further aspect,the polycarbonate production system is configured to perform thedisclosed processes. In a further aspect, the polycarbonate productionsystem comprises the disclosed caustic scrubbers and configured toperform the disclosed processes. In a further aspect, the polycarbonateproduction system comprises the disclosed caustic control systems. In afurther aspect, the polycarbonate production system is configured toperform the disclosed processes. In a further aspect, the polycarbonateproduction system comprises the disclosed caustic control systems andconfigured to perform the disclosed processes.

While aspects of the present invention can be described and claimed in aparticular statutory class, such as the system statutory class, this isfor convenience only and one of skill in the art will understand thateach aspect of the present invention can be described and claimed in anystatutory class. Unless otherwise expressly stated, it is in no wayintended that any method or aspect set forth herein be construed asrequiring that its steps be performed in a specific order. Accordingly,where a method claim does not specifically state in the claims ordescriptions that the steps are to be limited to a specific order, it isno way intended that an order be inferred, in any respect. This holdsfor any possible non-express basis for interpretation, including mattersof logic with respect to arrangement of steps or operational flow, plainmeaning derived from grammatical organization or punctuation, or thenumber or type of aspects described in the specification.

Throughout this application, various publications are referenced. Thedisclosures of these publications in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the state of the art to which this pertains. The referencesdisclosed are also individually and specifically incorporated byreference herein for the material contained in them that is discussed inthe sentence in which the reference is relied upon. Nothing herein is tobe construed as an admission that the present invention is not entitledto antedate such publication by virtue of prior invention. Further, thedates of publication provided herein can be different from the actualpublication dates, which can require independent confirmation.

EXAMPLES

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how themethods, devices, and systems disclosed and claimed herein are made andevaluated, and are intended to be purely exemplary and are not intendedto limit the disclosure. Efforts have been made to ensure accuracy withrespect to numbers (e.g., amounts, temperature, etc.), but some errorsand deviations should be accounted for. Unless indicated otherwise,parts are parts by weight, temperature is in degrees Celsius (° C.) oris at ambient temperature, and pressure is at or near atmospheric.

Example 1

A sample was obtained from the side loop of a caustic scrubber andanalyzed by titration. One gram of the sample was weighed into a 120 mlsample cup, and then 50 ml of distilled water was added. The sample wasthen titrated with 1 N hydrochloric acid (HCl) until the end point wasreached. Sampling and measurements were done about every 8 hours. Thismethod provides for discrete sampling without any additional informationon caustic concentration in scrubbers between the 8 hr samplingintervals. The total analysis time for titration is about 10 minutes.

At the same instant titration sample was being collected, threereplicate NIR spectral measurements were acquired via the probe in theside loop. These spectra were acquired before and after the sampling fortitration measurement by manually requesting a scan so that a comparisonof the caustic by titration and caustic by NIR could be realized. TheNIR spectrometer used in this example was a Guided Wave Process 412 NIRspectrometer (Rancho Cordova, Calif.) coupled to an O-ring single sided(O-SST) transmission probe via fiber optics. The pathlength of thetransmission probe was 2 mm and the instrument's resolution was 1 nm.The comparative study was carried out for two months. Replicate NIRmeasurements acquired were averaged for each time point. In total therewere 93 comparative data points.

In anticipation of analyzing caustic scrubber samples in the processstream, the Guided Wave NIR spectrometer was first calibrated on a setof standards with caustic concentrations that spanned the entire targetrange to be measured. The calibration curve obtained was then stored inthe computer controlling the spectrometer.

The acquired NIR spectra were then subjected to the chemometric basedPLS regression model for caustic which outputs the causticconcentration. Direct NIR measurement followed by subsequent computationof caustic concentration was done in process stream is non-destructive,without sample preparation and use of reagents. The NIR spectral scanranges from 1000-2100 nm. The spectral region from 1000-1800 nm is usedfor the PLS regression model.

The results for 93 comparative samples are shown in FIG. 2. Visualinspection clearly shows that the two measurement systems are similar.Statistical comparison based on paired t-test revealed the differencebetween the two measurement systems was not significant (p=0.38).However, the total analysis time for the online NIR is less than 1minute making it suitable for real-time analysis and process monitoring.

Example 2

The same spectrometer and probe configuration described in Example 1were used in this example. The transmission probe was inserted into theprocess stream via the side loop and measured the NIR signal of thecaustic flowing through the side loop. The same chemometric based PLSregression caustic model utilizing 1000-1800 nm was used to compute thecaustic concentration in real-time. Data acquisition and subsequentcaustic concentration output was programmed for once every 5 minutes for15 days, resulting in a the collection and analysis of 4,320 NIRspectra.

The results for the exemplary real-time online NIR analysis (graydiamond symbols in FIGS. 3A-3B) and comparison to titration results(black square symbols in FIG. 3A) are displayed in FIGS. 3A-3B. Visualinspection clearly shows the two measurement systems are comparable. Theresults in this example show that the caustic level can be controlled inreal-time with an improved cycle time and safer operation of the causticscrubbers.

Example 3

The same spectrometer and probe configuration as described in Example 1were used in this example. The transmission probe was inserted into theprocess stream via the side loop and measured the NIR signal of thecaustic flowing through the side loop. The same chemometric based PLSregression caustic model utilizing 1000-1800 nm was used to compute thecaustic concentration in real-time. Data acquisition and subsequentcaustic concentration output was programmed for once every 1 minute,corresponding to 1,440 spectra collected and analyzed.

Acidic gases emanating from the resin reactor and centrifuge feed tankreact with caustic in the caustic column and are neutralized. Theimmediate consequence of this is the concentration of caustic is reducedand eventually the neutralized caustic is pumped to the waste tank. Whenthe caustic level in the circulation tank falls below 70%, thisautomatically triggers the need to pump fresh caustic such that thelevel is brought up to 75%. This subsequently registers as an increasein caustic concentration. The data shown in FIG. 4 represent the causticconcentration over several weeks. FIG. 5 shows data pertaining to theperturbation of the caustic concentration within 24 hours.

The decline in the caustic concentration corresponds to neutralizationof the caustic by acidic gases while the increase in the causticconcentration is due to the recharging of the circulation tank. Thesedata clearly show the benefits of the present methods comprisingreal-time online monitoring of caustic levels in process scrubbers. Thisphenomenon was not evidenced when caustic monitoring and control waslinked to a discrete measurement scheme of titration.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the scope or spirit of the invention. Otherembodiments of the invention will be apparent to those skilled in theart from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal languages of the claims.

What is claimed is:
 1. A monitoring process comprising the steps of: (a) providing a spectroscopy probe coupled to a caustic scrubber; and (b) measuring the near-infrared absorbance in the scrubber.
 2. The process of claim 1, wherein coupling is via direct contact.
 3. The process of claim 1, wherein coupling is via fiber optic.
 4. The process of claim 1, wherein the probe is a NIR transmission probe.
 5. The process of claim 1, wherein caustic is sodium hydroxide.
 6. The process of claim 1, further comprising the step of determining caustic concentration in the scrubber from the measured absorbance.
 7. The process of claim 6, wherein the determining step is a chemometric-based method with spectral averaging over a time period of from about 15 seconds to about 5 minutes.
 8. The process of claim 1, wherein the caustic scrubber is part of a polycarbonate production system.
 9. The process of claim 1, further comprising the step of introducing caustic into the scrubber to maintain a concentration range of from about 5% to about 15%.
 10. The process of claim 9, wherein the concentration range is from about 6% to about 8%.
 11. The process of claim 9, wherein the caustic is introduced in response to the measured absorbance in accordance with a pre-determined set point value.
 12. The process of claim 10, wherein the caustic is introduced automatically into the scrubber.
 13. The process of claim 12, wherein the automatic introduction of caustic is in accordance with a pre-programmed algorithm.
 14. The process of claim 1, wherein the absorbance is measured from about 1000 nm to about 2000 nm.
 15. The process of claim 1, wherein the Reynolds Number of the fluid flow in the scrubber is less than
 1500. 16. The process of claim 1, wherein the Reynolds Number of the fluid flow in the scrubber is greater than
 4000. 17. A process for monitoring and controlling caustic concentration in a caustic scrubber of a polycarbonate production system, the process comprising the steps of: (a) coupling, via fiber optic, a NIR transmission probe to a caustic scrubber; (b) measuring the near-infrared absorbance from about 1000 nm to about 2000 nm in the scrubber; (c) determining caustic concentration in the scrubber from the measured absorbance via a chemometric-based method; and (d) introducing, in response to the measured absorbance, caustic into the scrubber to maintain a concentration range of from about 5% to about 15%.
 18. A caustic scrubber comprising a near-infrared spectroscopy probe coupled thereto.
 19. The scrubber of claim 18, wherein the probe is coupled to the scrubber via direct contact.
 20. The scrubber of claim 18, wherein the probe is coupled to the scrubber via fiber optic.
 21. The scrubber of claim 18, wherein the caustic scrubber is part of a polycarbonate production system.
 22. The scrubber of claim 18, wherein caustic is present in the scrubber within a concentration range of from about 5% to about 15%.
 23. The scrubber of claim 22, wherein the concentration range is from about 6% to about 8%.
 24. The scrubber of claim 18, further comprising a means to introduce caustic into the scrubber.
 25. The scrubber of claim 24, wherein the caustic is introduced in response to the measured absorbance to maintain the concentration range.
 26. The scrubber of claim 24, wherein the caustic is introduced in response to the measured absorbance in accordance with a pre-determined set point value.
 27. The scrubber of claim 25 or 26, wherein the caustic is introduced automatically into the scrubber.
 28. The scrubber of claim 28, wherein the automatic introduction of caustic is in accordance with a pre-programmed algorithm.
 29. A caustic control system in a polycarbonate production system, the scrubber comprising: (a) a caustic scrubber; (b) a spectroscopy probe coupled to the scrubber via fiber optic, wherein the probe is capable of measuring the near-infrared absorbance in the scrubber; (c) means for determining caustic concentration in the scrubber from the measured absorbance; and (d) means to automatically introduce caustic into the scrubber to maintain a concentration range of from about 5% to about 15%.
 30. A polycarbonate production system comprising the caustic scrubber of claim
 18. 31. A polycarbonate production system configured to perform the process of claim
 1. 32. A polycarbonate production system comprising the caustic scrubber of claim 18 and configured to perform the process of claim
 1. 33. A polycarbonate production system comprising the caustic control system of claim
 29. 34. A polycarbonate production system configured to perform the process of claim
 17. 35. A polycarbonate production system comprising the caustic control system of claim 29 and configured to perform the process of claim
 17. 